1. Field of the Invention
The present invention relates to an image forming apparatus including a skew correction mechanism, a control method therefor, and a storage medium, and more particularly, to an image forming apparatus including a skew correction mechanism that corrects skew of a sheet including a tabbed sheet, a control method therefor, and a storage medium.
2. Description of the Related Art
Some conventional image forming apparatuses such as copiers, printers, or facsimile machines include a skew correction mechanism that corrects skew of a sheet so as to align a direction or a position of a sheet conveyed to an image forming unit.
A skew correction method of a sheet in an image forming apparatus includes a method of correcting skew by forming a loop in a sheet using a pair of registration rollers. However, in this skew correction method, temporarily stopping conveyance of the sheet requires time to convey the sheet to a transfer position. In order to reduce the time required for the conveyance, an active registration method has been proposed of using two sensors and two sets of skew correction rollers independently rotated to convey and rotate a sheet, thereby correcting skew (see Japanese Patent Laid-Open Patent Publication (Kokai) No. 10-032682).
In the active registration method, first, a leading edge of the sheet is detected based on detection signals from the two sensors provided on a conveying path of the sheet in a direction perpendicular to a conveying direction when the leading edge of the sheet crosses the sensors. Then, a skew amount of the sheet is detected based on a difference in generation timing of the detection signals from the two sensors. Then, rotational speeds of two drive motors that drive the two sets of skew correction rollers respectively lying on either side of the conveying path are controlled depending on the detected skew amount, and sheet conveying speeds of the two sets of skew correction rollers are changed depending on the skew amount of the sheet, thereby correcting skew of the sheet. Specifically, in skew correction by the active registration method, depending on the skew amount of the sheet, the rotational speed of one skew correction roller is set lower (skew speed reducing control) or higher (skew speed increasing control) than the rotational speed of the other skew correction roller to correct skew of the sheet.
In the skew correction by the active registration method, the skew correction is performed without stopping conveyance of sheets temporarily, and thus a sheet interval (an interval between a preceding sheet and a following sheet) can be shorter than by other methods. This can increase sheet conveying efficiency, and for example, increase a substantial image forming speed without increasing an image forming process speed of an image forming apparatus. Thus, the active registration method is adopted in an image forming apparatus as a skew correction method that contributes to increase a speed of an image forming operation.
In recent years, demands for image forming on sheets having various shapes have been increased, and it has been desired to pass sheets not always having a rectangular shape, particularly, tabbed sheets in an image forming apparatus. The tabbed sheet refers to a sheet having, on an end side, a tab in which an index or the like is written for classification. The tab provided at a sheet end is not provided in a fixed position but in various positions so that indexes such as letters or figures written in the tabs can be easily checked. Thus, for skew correction of tabbed sheets, a method has been proposed of obtaining position information of a tab, and depending on whether a sensor detects the tab, correcting information from the sensor by an amount of a tab width to perform skew correction (see Japanese Patent Laid-Open Patent Publication (Kokai) No. 2003-146485).
The sheet having been subjected to the skew correction is controlled in conveying speed by registration rollers so that a leading edge of the sheet reaches a toner image transfer position of an image forming unit at predetermined timing. Specifically, when the sensor placed on a downstream side of the registration rollers on a conveying path detects the leading edge of the sheet, the sensor outputs a detection signal to a control unit, the control unit controls a rotational speed of the registration rollers depending on the detection signal to synchronize the sheet with a toner image on a photoconductive drum. Thus, the toner image is formed in an appropriate position on the sheet.
In the image forming apparatus, in order to convey a tabbed sheet having a tab on a leading edge side in a sheet conveying direction to a toner image transfer position at appropriate timing, the leading edge of the tabbed sheet needs to be detected except the tab after skew correction of the tabbed sheet is completed. Thus, there is a method of placing a plurality of sensors in a direction perpendicular to a sheet conveying direction on a downstream side of a pair of skew correction rollers, and precisely detecting a sheet leading edge except a tab by a difference in detection timing of the sensors.
However, when skew correction is performed by the above conventional method, the sheet leading edge needs to be detected by the sensors after the skew correction of the sheet is completed. This increases a sheet conveying path, and increases a size of an apparatus.